Seeds are commonly treated with pesticides to control insects, nematodes, and disease organisms such as fungi and bacteria. Neonicotinoid compounds are commonly active ingredients of insecticides used for treating seeds. Among the commercially available neonicotinoid compounds are clothianidin (tradename Poncho®), imidacloprid (tradename Gauche®), thiamethoxam (tradename Cruiser®), and dinotefuran (tradename Safari®). N-(2-phenylethyl)succinamic acid is root growth promoter (Soejima, H., et al., Plant Cell Physiol., 2000, 41, p. 197; Itagaki, M., et al., 6th Symposium of the International Society of Root Research, 2001, C1-8; Itagaki, M., et al., Plant Soil, 2003, 255, p. 67-75). Effects of combinations of neonicotinoid compounds and PESA or its salts on plant growth have not been previously examined.